1. Field of the Invention
This invention relates to a polymeric solid electrolyte and electrode for use in lithium secondary cells and electric double-layer capacitors, and a lithium secondary cell, electric double-layer capacitor and EL device using the polymeric solid electrolyte and electrode.
2. Background Art
Secondary batteries for use in notebook size personal computers, video cameras, and cellular phones are required to have a high capacitance and a satisfactory charge/discharge cycle life. The secondary batteries which have been used heretofore include lead acid batteries, nickel-cadmium batteries, and nickel-hydrogen batteries. Lithium secondary batteries have been used in practice as a smaller size, higher capacitance battery.
For example, JP-A 121260/1988 discloses a secondary cell using a non-aqueous electrolytic solution, LiCoO.sub.2 and/or LiNiO.sub.2 as a positive electrode, and carbon as a negative electrode. At the negative electrode in this secondary cell, for example, lithium ions enter between layers of hexagonal net planes of carbon to receive electrons during charging while lithium between carbon layers is ionized again by releasing electrons. The lithium secondary cell of this type, which is known as a Li-ion secondary cell, provides a high capacitance for a small size and is safe due to the eliminated need for highly reactive metallic lithium. There is a strong desire to further improve its performance.
The electrode used in lithium secondary cells is manufactured by joining an active material such as carbon material powder and lithium composite oxide powder to a surface of a current collector in the form of a copper or aluminum foil. More particularly, the active material powder is dispersed in a binder solution and then coated onto the current collector surface while the binder must meet the following requirements.
(1) The binder has a sufficient bond strength to prevent a coating of active material from stripping from the current collector and cracking in a battery assembling process.
(2) The binder is not soluble in an electrolytic solution.
(3) The binder has a sufficient bond strength to prevent a coating from stripping from the current collector and cracking upon repetitive charge/discharge cycles.
(4) A small amount of the binder added provides sufficient bond strength.
(5) The binder is not oxidized or reduced in the operating voltage range.
(6) The binder is not soluble in an organic solvent for use in an electrolytic solution, but soluble in a solvent for use in coating to the current collector surface.
In the above-referred JP-A 121260/1988, polyvinylidene fluoride (PVDF) is used as the binder. However, a large amount of polyvinylidene fluoride must be added in order to increase bond strength although this tends to invite insufficient cell capacitance because the binder does not contribute to cell capacitance. Another problem is that since polyvinylidene fluoride is soluble in an non-aqueous solvent of an electrolytic solution, repetitive charge/discharge cycles can cause stripping and cracking of the coating to reduce the cell capacitance even when polyvinylidene fluoride is added in large amounts.
Also, PVDF causes many troubles in an actual battery manufacturing process since it is a crystalline resin. In one exemplary process, a coating solution of active material dispersed in a PVDF solution is coated onto a current collector (e.g., copper foil) and dried to form an electrode. If a drying rate and other factors are inadequate in this process, undesirably the electrode mix layer can strip from the current collector or curl even if it does not strip because of a substantial difference in shrinkage factor between PVDF and the current collector. Even when no problems are found immediately after coating and drying, there is a likelihood that the electrode mix layer gradually strip from the current collector with the lapse of time owing to the internal stresses remaining in the electrode. Furthermore, JP-B 4007/1996 proposes fluorinated high molecular weight copolymers including PVDF, which also suffer from problems similar to the PVDF.
Besides, a number of other compounds have been used as the binder. Of these compounds, preferred examples are crosslinkable polymers. Included are polymers which are crosslinked using polyamines, polyols or peroxides as a crosslinking agent, for example, at least one of vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers, tetrafluoroethylene-propylene copolymers, tetrafluoroethylene-propylene-vinylidene fluoride copolymers, and perfluoro fluororubbers crosslinked with the above-mentioned crosslinking agent. The amount of the crosslinking agent added is generally about 0.5 to 10 parts, preferably about 1 to 5 parts by weight per 100 parts by weight of the compound to be crosslinked.
Also included are fluorine-containing compounds which are crosslinked with radiation such as .beta.-rays and .gamma.-rays, for example, at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymers, and fluorinated thermoplastic rubbers crosslinked with radiation. These compounds are described, for example, in Journal of the Japanese Chemical Society, No. 4, 686 (1976) and Industrial and Engineering Chemistry, Vol. 49, No. 10, 1687 (1957).
It is also proposed to use a silane-crosslinked polyvinylidene fluoride which is obtained by grafting a silane compound to polyvinylidene fluoride followed by crosslinking with water. The silane-crosslinked polyvinylidene fluoride is described, for example, in JP-A 115234/1990.
Allegedly the crosslinked polymers may be used as a mixture of two or more. In addition to the above-mentioned crosslinked polymers, the binder may contain another polymer such as polymethyl methacrylate (PMMA) and polycarbonate (PC). The other polymer is contained in an amount of less than about 25% by volume of the entire binder.
While the above-mentioned crosslinked polymers are preferred binders, they require an extra crosslinking step before use in practice, imposing a problem to the manufacture process.
Most of currently commercially available batteries use as the electrolyte a so-called electrolytic solution obtained by dissolving an electrolyte salt in a liquid solvent. Batteries using electrolytic solution have the advantage of low internal resistance while they suffer from the problems of frequent solvent leakage and potential ignition. In view of these problems, research has long been made on solvent-free electrolytes, that is, solid electrolytes. For example, systems having electrolyte salts dissolved in polymers are known. It is noted that such solid electrolytes which are completely free of solvents (for example, lithium salt dissolved in polyethylene oxide) have not reached a practical level because of a low conductivity of less than 10.sup.-4 S/cm. In contrast, gel-like polymeric solid electrolytes comprising a polymer, an electrolyte salt, and a solvent have been highlighted in the recent years.
Such gel-like polymeric solid electrolytes, which are referred to as "gel electrolytes" hereinafter, have a conductivity close to liquids and some mark a conductivity of the order of 10.sup.-3 S/cm.
For example, U.S. Pat. No. 5,296,318 discloses a gel electrolyte containing 20 to 70% by weight of a lithium salt solution in a copolymer P(VDF-HFP) of vinylidene fluoride (VDF) with 8 to 25% by weight of hexafluoropropylene (HFP). This gel electrolyte has a conductivity up to 10.sup.-3 S/cm. Originally, polyvinylidene fluoride (PVDF) is a crystalline polymer having relatively high chemical resistance. Namely, PVDF are well soluble in some solvents, but not soluble in every solvent. Among fluoro-resins, PVDF is one of easy-to-use resins. In fact, PVDF is used as a binder for positive and negative active materials in commercially available Li-ion secondary batteries. The PVDF described in the above-referred patent is a copolymer of VDF and HFP wherein HFP lowers the crystallinity of PVDF. These VDF-HFP copolymers can retain a large amount of solvent, inhibit the precipitation of lithium salt crystals, and ensure the formation of gel electrolyte having strength.
Despite the above-mentioned possibility to achieve a high conductivity, the VDF-HFP copolymer, in which HFP lowers the crystallinity of PVDF, has the drawbacks of chemical resistance and melting point drops inherently involved in such a polymer structure. For example, PVDF homopolymers commercially available from Elf Atochem (trademark, KYNAR 700 series) have a melting point of 170.degree. C. whereas a VDF-HFP copolymer also commercially available from Elf Atochem, for example, KYNAR 2801 has a melting point of 145.degree. C. Since the chemical resistance drop means that the copolymer is more soluble in the cell electrolytic solution, a cell using such a VDF-HFP copolymer is low in storage properties. For example, when the cell is stored at room temperature or elevated temperatures of 40.degree. C., 60.degree. C., 80.degree. C. and 100.degree. C., there occur capacitance losses and in an extreme case, internal short-circuit. The melting point drop restricts the use at high temperature and leads to poor storage properties at high temperature as mentioned above.
Furthermore, in a cell using a gel electrolyte as in U.S. Pat. No. 5,296,318, for example, an electrode is reduced in interfacial resistance by using a composition of electrode active material and a gel electrolyte to improve the adhesion between the electrode and the gel electrolyte. The electrode disclosed in this patent, however, is less adhesive to the current collector to be coated therewith, leaving the possibility that the electrode mix portion strip after coating. The PVDF separates from the current collector because PVDF has a substantial shrinkage factor upon coating and drying due to the crystalline resin nature. This means that conductivity becomes increased whereas adhesion to the current collector is insufficient. For this reason, when the gel electrolyte described in this patent is used, the electrode formation is not achievable by a simple coating step and the continuous lamination of coating films is difficult. Thus, an electrode is constructed using metal mesh as a support. Another device using the above-mentioned binder material as an electrode material is an electric double-layer capacitor. When the binder material is used as a polarizing electrode material in an electric double-layer capacitor, it is also desired to improve the properties of the binder material as in the case of lithium secondary cells. Better gel electrolytes are demanded for the electric double-layer capacitor too.
Also known is an EL device comprising a light emitting layer containing a fluorescent material and a binder. The development of a binder material for the EL device is also desired.